In recent years, mechanisms for quality control of nascent proteins based on interactions between sugar chains and intracellular lectin/molecular chaperones have been drawing attention as a subject of active research worldwide in the field of sugar chain biology. In the rough endoplasmic reticulum, the majority of proteins are modified by a tetradecasaccharide consisting of three glucose units, nine mannose units, and two N-acetylglucosamine units (Glc3Man9GlcNAc2). Such protein-bound sugar chains are decomposed by glycosidases, resulting in the formation of a dodecasaccharide (GlcMan9GlcNAc2; see below). This is followed by trimming of the sugar chain moieties by glycohydrolases; it is postulated that there exists a lectin-like protein that accurately recognizes the various sugar chain structures and polypeptide moieties resulting in this trimming process, which protein acts to deliver other proteins in the correct higher-order structure to Golgi body, to retain immature proteins in the endoplasmic reticulum until they assume the correct higher-order structure, and to direct faulty proteins failing to assume the correct higher-order structure to endoplasmic reticulum-related decomposition. Hence, it is considered that in the endoplasmic reticulum, protein quality control takes place through confounding interactions between sugar chains and glucosidases, glucose transferases, the lectin-like protein and the like.
The Structure of High Mannose Type Sugar Chain (GlcMan9GlcNAc2)
The Schematic Structure of High Mannose Type Sugar Chain (GlcMan9GlcNAc2)

Currently, thanks to technical improvements in sugar chain separation and analysis, an increasing variety of sugar chain samples are available from naturally occurring products. It is difficult, however, to purify from a natural product a desired sugar chain (e.g., the sugar chains resulting from glycosidase decomposition of high mannose type sugar chain, shown below) in a sufficient amount to enable its application to sugar chain functional analysis, or to various products and methods. Hence, there is a demand for the development of a method that enables easier obtainment of various naturally occurring sugar chains (sugar chain library).
The Structures of Naturally Occurring Sugar Chains Generated from High Mannose Type Sugar Chain (GlcMan9GlcNAc2)

Some methods using enzymes are known to date for preparing a sugar chain library. For example, Kajihara et al. (Chemistry—A European Journal Vol. 10: 971-985 (2004)) describes a method of preparing a sugar chain library using enzymes, which comprises converting an asparagine-bound sugar chain to various sugar chains by repeating limited degradation and sugar chain elongation reaction using a glycosidase and a glycosyl transferase in combination. However, these methods are problematic in that various naturally occurring sugar chains from high mannose type sugar chain like those described above cannot be produced specifically and systematically because, if the starting sugar chain compound has, for example, a plurality of non-reducing termini having the same kind of sugar residue (D-mannose) bound to the adjacent sugar residue in the same binding mode (α1-2 linkage), as in the above-described high mannose type sugar chain, the same reaction occurs undesirably in the plurality of non-reducing termini even in the presence of a glycosidase and a glycosyl transferase.